A bushy neuron in the cochlear nucleus, with a glass
microelectrode for recording electrical activity inside the cell.
New research shows that the synapses onto these cells are grouped
by plasticity. Photo: L. Pliss.

BUFFALO, N.Y. -- The brain receives information from the ear in
a surprisingly orderly fashion, according to a University at
Buffalo study scheduled to appear June 6 in the Journal of
Neuroscience.

The research focuses on a section of the brain called the
cochlear nucleus, the first way-station in the brain for
information coming from the ear. In particular, the study examined
tiny biological structures called synapses that transmit signals
from the auditory nerve to the cochlear nucleus.

The major finding: The synapses in question are not grouped
randomly. Instead, like orchestra musicians sitting in their own
sections, the synapses are bundled together by a key trait:
plasticity.

Plasticity relates to how quickly a synapse runs down the supply
of neurotransmitter it uses to send signals, and plasticity can
affect a synapse's sensitivity to different qualities of sound.
Synapses that unleash supplies rapidly may provide good information
on when a sound began, while synapses that release neurotransmitter
at a more frugal pace may provide better clues on traits like
timbre that persist over the duration of a sound.

UB Associate Professor Matthew Xu-Friedman, who led the study,
said the findings raise new questions about the physiology of
hearing. The research shows that synapses in the cochlear nucleus
are arranged by plasticity, but doesn't yet explain why this
arrangement is beneficial, he said.

"It's clearly important, because the synapses are sorted based
on this. What we don't know is why," said Xu-Friedman, a member of
UB's Department of Biological Sciences. "If you look inside a file
cabinet and find all these pieces of paper together, you know it's
important that they're together, but you may not know why."

In the study, Xu-Friedman and Research Assistant Professor Hua
Yang used brain slices from mice to study about 20 cells in the
cochlear nucleus called bushy cells, which receive information from
synapses attached to auditory nerve fibers.

The experiments revealed that each bushy cell was linked to a
network of synapses with similar plasticity. This means that bushy
cells themselves may become specialized, developing unique
sensitivities to particular characteristics of a sound, Xu-Friedman
said.

The study hints that the cochlear nucleus may not be the only
part of the brain where synapses are organized by plasticity. The
researchers observed the phenomenon in the excitatory synapses of
the cerebellum as well.

"One reason this may not have been noticed before is that
measuring the plasticity of two different synapses onto one cell is
technically quite difficult," Xu-Friedman said.